Heretofore, wear-resistance, machinability and toughness of cold tool steels have been improved, indeed. However, in order to cope with the recent cold dies, requiring cast saving, fast delivery and precise size, the most important problem, that is to minimize the size-change contingent to the heat-treatments, especially to hardening or tempering, has not bee solved, as yet.
Among various heat-treatment deformations, the size-change caused by wrong cutting direction, warping and/or twisting can be solved by reasonable measurements. However, said size-change contingent to hardening or tempering cannot be avoided, because said change will be caused by thermal stress and transformation-stress, and the extent of such change depends upon cooling velocity, elasticity limit, heat conductivity, residue of austenite, carbides, and configuration of the dies material, itself.
Jpn. Pat. Publn. No. H.4-116122 describes that the machinability has been improved remarkably by dispersion of carbides and metallographic uniformity attained by controlling the treat-temperature between 1150-900.degree. C., and the draft-ratio at 3 or over, as well as by controlling the quantity of carbon and of carbonizing elements. These results were achieved from the point of view that the uniformity of metallographic organization around the primary carbide is important for machinability, toughness and brittle-proofing, it arranges.
Also Jpn. Pat. Publn. No. H.8-120333 discloses the manufacturing method of cold tool steel being provided with effective wear-resistance, machinability and toughness.
Further, Jpn. Pat. Publn. S.56-16975 reveals to provide steels with good quenching property and thermal deflection, both through finding the important rolls of the quantity of aluminum and nitrogen in the steel in the manufacturing of desired steel, both elements of which have not so far been considered to have such roles.
None of the prior arts, however, have achieved to minimize the size-change contingent to hardening and/or tempering.
On the other hand, some steels, as Jpn. Pat. Publn. Nos. S.63-183185 & S.52-1372 indicates, were cut directly after pre-hardening and used as dies mainly for plastics.
Because of being free of deformation of scale, said pre-hardened steels were advantageous as regards delivery and cost.
However, said steels had relatively low hardness, ranging 10.about.45 HRC, and therefore were not used for highly wear-resistant press-molds or punches to which JIS SKD 11 is applied. The reason for it is that the machinability of said steels was very low in the state of higher hardness beyond 55 HRC, and in turn, their wear-resistance was so poor, in the lower hardness state, that they could not be used practically.
In order to resolve various problems as mentioned above, the present inventors propose, as described in Jpn. Pat. Publn. No. H.8-120333 and Jpn. Pat. Publn. No. H.9-268010, such cold tools steels as having the improved machinability contingent to heat-treatment, wherein, the optimum components and the percentage thereof for minimizing the size-change contingent to hardening and tempering were determined through repeated investigations and analyses.
Another object of the present invention is to provide, combining said inventions as mentioned above, such cold tool steels as having excellent toughness wear-resistance, and high-machinability even at the hardness as high as 55.about.60 HRC, all of which are superior than those as specified for JIS SKD 111.